Lighting systems for landing in a degraded visual environment

ABSTRACT

Lighting systems and methods for landing in a degraded visual environment are disclosed. The lighting systems comprise one or more lighting units mounted to an aircraft operable to hover near a landing zone. Each lighting unit is operable to provide adjustable illumination to the landing zone, and has a radiant power output between a minimum and a maximum. The minimum radiant power output is just sufficient to allow a pilot to distinguish features in the landing zone when below a first altitude wherein the downwash from the aircraft rotors, propellers, or engines begins to raise particulates from the landing zone and continues to be just sufficient to allow the pilot to distinguish features as the pilot descends from the first altitude to termination at the landing zone, and the maximum radiant power output is less than about five times the minimum radiant power output.

FIELD OF THE INVENTION

One or more embodiments of the present invention relate to illuminationsystems for assisting in landing aircraft in a degraded visualenvironment.

BACKGROUND

Rotorcraft (e.g., helicopters) and VTOL (vertical takeoff and landing)aircraft have a natural tendency to stir particulates into the air withtheir downwash when operating near the Earth's surface. This typicallyoccurs while maintaining or transitioning in or out of the hoveringflight regime, at an altitude that is within one to two times theequivalent rotor diameter. In certain environments, a relatively highconcentration of stirred particulates, typically dust, may significantlyobscure the pilot(s) field of view (FOV). The particulates can result inthe loss of outside visual references, or brownout (in a dustenvironment), which can induce spatial disorientation in a pilot. Thepilot(s) may not be able to see the ground. Depending on conditions,particulates may comprise dust, sand, snow, or other materials that canbecome airborne either due to rotor downdraft or local weatherconditions.

The difficulty of a brownout situation is further compounded at night,because there are fewer visual cues available. Further, pilots(especially military aviators) may be using night-vision goggles (NVGs)which typically restrict the FOV to 40°. Using NVGs, also referred to asflying “aided,” provides a visual acuity of 20/25 at best [Army 2007A].Due to the challenges involved in such operations, pilots learn to use avariety of compensatory methods.

One method of compensation is to maintain enough forward airspeed duringthe approach and touchdown to outrun the formation of particulates andprevent the particulate formation from enveloping the cockpit. Whileeffective in some situations, this method is not suitable for manylanding zones, particularly those that are rough, sloping, confined, orpinnacle. Another method is referred to as termination to a point OGE(out of ground effect) [Army 2007B, Army 2013]. This method requiresmore power. The initial approach is to a high hover position directlyover the intended point of landing. The high hover position is used tostir and dissipate the dust before descending to the ground. HoveringOGE is effective is some situations, but there are disadvantages. First,depending on the aircraft gross weight and environmental conditions, thepower required may not be available to hover OGE. Second, it may not bea tactically advisable maneuver, because it exposes the aircraft in itsmost vulnerable state for an extended period. Third, descending from anOGE hover surrounded by a ring of circulating dust can induce spatialdisorientation, resulting in improper control manipulation, consequentaircraft drift, and/or an unanticipated, possibly damaging touchdownrate.

Generally, in helicopters, once outside references are lost out of thewindshield, focus is directed through the chin bubble and/or othercockpit door windows. If references are lost through the chin bubble andwindows, the focus transitions to the flight instruments, and theapproach is aborted using an instrument take-off (ITO). Technically,since the maneuver is not initiated from the ground, it is actually morea modified “go-around.” Once above the dust with adequate visibility,the crew may continue visually and re-evaluate the situation.

Regardless of the method employed, there is always the potential tobecome partially or completely enveloped in dust. At night, whenshifting focus from the windshield to the chin bubble or other windows,NVG use provides additional limitations. When looking through NVGs,depth perception is severely limited, especially at close ranges. Theresolution at close range may be lower due to individual focus settingsor constraints. The pilot may not have a clear sight picture of theimmediate ground surface during the final stage of an approach.Cross-checking the chin bubble or cockpit door window looking throughNVGs requires a large head movement that can be hazardous during thecritical final moments of an approach.

An alternative, not printed in Army training manuals, is used in somecases to provide improved visibility beneath the aircraft. The landinglight or search light is turned on, and the pilot looks beneath the NVGeyepieces and through the chin bubble or cockpit door window using theunaided eye. This technique can offer the best combination of availableoptions by allowing the pilot to divide his/her attention by lookingthrough the windshield using the NVGs (arrow 102) at horizon associatedreferences and maintaining a good ground reference cross-check byglancing through the chin bubble unaided (arrow 104), as illustrated inFIG. 1.

One problem with using landing lights or search lights in this way isthat tactical considerations may be sacrificed to the intensity of thelight. Further, the landing light and searchlight are consideredincompatible with NVGs, because they are conventional white lights(unless the searchlight is infrared). The compatibility issue is,however, more of a misconception than a reality with modern NVGs. ModernNVGs, such as the AN/AVS-6(V)3 (Exelis Night Vision, Roanoke Va.), haveautomatic brightness control (ABC) and bright source protection (BSP)which are built-in features designed to prevent blinding the user ordamaging the NVGs. However, most white lights are still not conducivefor use with NVGs, because ambient light is amplified approximately2000-3000 times by the goggles, and BSP has the side effect of loweringresolution [Army 2007A].

Viewing underneath the goggles aided by the landing light or searchlightlight works in many instances, but in heavy or severe dust the pilot maystill be disoriented due to the intensity. This is likely why the methodis not formally recommended. Military helicopters do have infraredsearchlights that are considered NVG compatible, but as discussed above,looking through the chin bubble with NVGs requires excessive headmovement and provides low resolution viewing, limited FOV, and lack ofdepth perception. The intensity of the infrared searchlight can alsoproduce disorientation while looking through NVGs in heavy or severedust just as the landing light can. Although the infrared searchlighthas adjustable brightness, it is a very coarse adjustment, has limiteddirectional control, and always defaults to maximum brightness whenpower to it is cycled.

SUMMARY OF THE INVENTION

Lighting systems and methods for landing in a degraded visualenvironment are disclosed. The lighting systems comprise one or morelighting units mounted to an aircraft that is operable to hover near alanding zone. Each lighting unit is operable to provide adjustableillumination to the landing zone, and has a radiant power output betweena minimum and a maximum. The minimum radiant power output is justsufficient to allow a pilot to distinguish features in the landing zonewhen below a first altitude wherein the downwash from the aircraftrotors, propellers, or engines begins to raise particulates from thelanding zone and continues to be just sufficient to allow the pilot todistinguish features as the pilot descends from the first altitude totermination at the landing zone. The particulates can include dust,sand, water, snow, or ice.

The maximum radiant power output is less than about five times theminimum power. In some embodiments the maximum radiant power output isless than about three times the minimum radiant power output. In someembodiments the maximum radiant power output is less than about twotimes the minimum radiant power output.

Each lighting unit can include a plurality of lighting elementscollectively emitting light at a plurality of wavelengths. The pluralityof wavelengths produce white, green, and infrared illumination; eachillumination can be individually enabled. The plurality of lightingelements can be light emitting diodes (LEDs).

The illuminated area of the landing zone can be approximately equal insize and shape to the field of view of a pilot or crew member lookingthrough a chin bubble or a side door or window of the aircraft. A shroudcan be provided, limiting the emission of light to a direction towardthe illuminated area of the landing zone.

Each lighting unit further can also include one or more lasers aimedgenerally in the direction of the illuminated area of the landing zone.The lasers can emit light at a plurality of wavelengths including red,green, and infrared which can be individually enabled. The lasers can besemiconductor lasers each having a power output of between 0.1 mW and1.0 mW.

One or more lasers can create a pattern on the landing zone whichchanges as a function of altitude. A spot projected from a laser ontothe landing zone can change size as a function of altitude. A spotprojected from a laser onto the landing zone can change shape as afunction of altitude. Two spots projected from a laser onto the landingzone can have a separation which changes as a function of altitude.

A method for illuminating a landing zone is also provided. One or morelighting units are attached to an aircraft that is operable to performvertical takeoffs and landings. Each lighting unit is operable toprovide adjustable illumination to a landing zone, and has a radiantpower output between a minimum and a maximum, wherein the minimumradiant power output is just sufficient to allow a pilot to distinguishfeatures in the landing zone from below a first altitude wherein theaircraft rotors, propellers, or engines begins to raise particulatesfrom the landing zone and continues to be just sufficient to allow thepilot to distinguish features as the pilot descends from the firstaltitude to termination at the landing zone, and wherein the maximumradiant power output is less than about five times the minimum radiantpower output. In some embodiments the maximum radiant power is less thanabout three times the minimum radiant power output. In some embodimentsthe maximum radiant power is less than about two times the minimumradiant power output. During an approach or landing of the aircraft, thelighting units are left off until the aircraft descend below the firstaltitude, and then turned on below a second altitude wherein the secondaltitude is below the first altitude. The second altitude can be about20 ft.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the “cross-check” technique looking through and under NVGs.

FIG. 2 shows aircraft losses from non-combat causes for 2002-9.

FIG. 3 shows simulated airflow (3A) and dust density (3B) for athrust-normalized advance ratio of 0.80.

FIG. 4 shows simulated airflow (4A) and dust density (4B) for athrust-normalized advance ratio of 0.29.

FIG. 5 shows simulated airflow (5A) and dust density (5B) for athrust-normalized advance ratio of 0.12.

FIG. 6 shows a concept illustration for a dust-light system mounted on ahelicopter.

FIG. 7 shows various views of a dust-light prototype

FIG. 8 shows an example mounting structure and connections

FIG. 9 shows the chin bubble FOV for the right pilot in a Black Hawkhelicopter

FIG. 10A shows the surface visible through the chin bubble; FIG. 10Billustrates the inverse square law.

FIG. 11 shows two proposed mounting locations for dust-lights on a BlackHawk helicopter.

FIG. 12 shows a first mounting location with the access panel removed.

FIG. 13 shows details of a second mounting location.

FIG. 14 shows a third mounting location.

FIG. 15 shows an example switch arrangement on the collective.

FIG. 16A shows the cockpit view during heavy dust approach at 19 ftabove ground level (AGL).

FIG. 16B shows the cockpit view during heavy dust approach at 9 ft AGL.

FIG. 17A shows the cockpit view during heavy dust approach at 3 ft AGL.

FIG. 17B shows the cockpit view during heavy dust approach attermination.

DETAILED DESCRIPTION

Before the present invention is described in detail, it is to beunderstood that unless otherwise indicated this invention is not limitedto specific aircraft or specific lighting modalities. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only and is not intended to limit thescope of the present invention.

It must be noted that as used herein and in the claims, the singularforms “a,” “and” and “the” include plural referents unless the contextclearly dictates otherwise. Thus, for example, reference to “a lightingunit” includes two or more lighting units, and so forth.

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit unlessthe context clearly dictates otherwise, between the upper and lowerlimit of that range, and any other stated or intervening value in thatstated range, is encompassed within the invention. The upper and lowerlimits of these smaller ranges may independently be included in thesmaller ranges, and are also encompassed within the invention, subjectto any specifically excluded limit in the stated range. Where the statedrange includes one or both of the limits, ranges excluding either orboth of those included limits are also included in the invention. Wherethe term “about” is used in front of a numerical value, the value isdeemed to be within ±10% of the numerical value.

As used herein, the term “particulate” refers to any small particlesthat can become airborne in a landing location either due to rotordownwash or due to local weather conditions. Examples include dust,sand, and snow. Examples described herein are general described usingdust as an exemplary but non-limiting embodiment of particulates thatcan reduce visibility. Exemplary landing zones are dry, dusty surfaces,although degraded visible environments can also occur under conditionsof heavy fog, rain, snow, water landings, marsh landings, and so on.

As used herein, the term “light dust” refers to a degraded visualenvironment (DVE) in which objects and terrain beyond the region of aparticulate cloud are visible (i.e., an observer such as a pilot can seethrough the DVE).

As used herein, the term “moderate dust” refers to a degraded visualenvironment (DVE) in which only objects and terrain within a particulatecloud are visible (i.e., an observer can see part way through the DVE).

As used herein, the term “heavy dust” refers to a degraded visualenvironment (DVE) in which no objects or terrain within a particulatecloud is visible (i.e., visibility ends abruptly within the DVE).

As used herein, the term “altitude” refers to the vertical position ofan aircraft (height above the Earth's surface) relative to a positionwhere the aircraft is stationary on the Earth's surface.

As used herein, the term “termination” refers to the end state of alanding approach where the “altitude” as defined above has reached zero.

The inventor has surprisingly found that pilots can perform best whenusing a combination of aided and unaided viewing techniques in the dustenvironment at night. Accordingly, there is a need for a purpose-builtlanding illumination system better suited to night-time landing inparticulate environments than prior art landing lights.

FIG. 2 shows the relative incidence of rotary wing combat non-hostileevent losses for the years 2002-9 from a study on rotorcraft safety[Harrington 2010]. “Brownouts” or “degraded visual environment” (DVE),accounted for the greatest number of combat non-hostile losses. Thestudy concluded that an integrated systems approach would be required toovercome the hazards associated with dust environments, suggestingelements such as improved cockpit display symbology and auto-landcapabilities.

Currently, such prospective solutions and avionics-based technologiesare in different stages of development. Various sensors are used toconstruct terrain and obstacle imagery displayed inside the cockpitalong with corresponding flight symbology. One approach usesmillimeter-wave radar to detect obstacle hazards and overlay them onto aknown terrain database. Another system uses ladar combined with adynamic graphics generator to produce a comparable display interface.While showing potential as integrated brownout landing solutions,workload may be very high for the pilot for obstacle avoidance. One orboth pilots' attention needs to be on the new display during the finalstages of approach and landing—a departure from conventional methods.The methodology associated with these systems introduces new humanfactors hazards, and certainly will require formal training andacclimation. These systems will require extensive integration intoexisting platforms at a considerable cost, particularly on a large-scaleretrofit. The required hardware for radar and ladar systems can weighbetween 30 and 100 pounds [Harrington 2010].

Researchers from the University of Glasgow conducted a simulation ofhelicopter brownout using fluid dynamics software to model variousparticle properties and their reactions under different flightconditions for both single rotor and tandem rotor configurations. Theycompared dust simulation results using varying values of the“thrust-normalized advance ratio” [Phillips 2009]. This important termaccounts for main rotor thrust, approach speed, and descent rate.Essentially, the larger the value, the higher forward airspeed is duringthe approach. FIGS. 3-5 illustrate that the predicted dust formationgains height and is more pronounced ahead of the aircraft as theapproach closure rates become slower.

The images in FIGS. 3A, 4A, and 5A show the three-dimensional flowpattern, and the images in FIGS. 3B, 4B, and 5B show the relativecross-sectional dust density. FIGS. 3A & 3B are for a thrust-normalizedadvance ratio of 0.29. FIGS. 4A & 4B are for a thrust-normalized advanceratio of 0.80. FIGS. 5A & 5B are for a thrust-normalized advance ratioof 0.12. As the forward speed of the aircraft decreases (lower advanceratio), a larger cloud of dust is raised. Note that the area directlybeneath and in front of the lower nose section (chin bubble area) has avery low relative dust density in all cases, suggesting that this can bea key area in which visual contact with the ground can be maintainedgiven a suitable illumination method.

Embodiments of the present invention provide improved illumination toaid pilots in landing rotorcraft and VTOL aircraft in particulate (lightdust to heavy dust) landing conditions, especially under limited lightconditions such as night-time landings and where tactical considerationsrequire the use of low light levels to minimize visibility to outsideobservers. As will be detailed below, the lighting systems can havemultiple modes and levels to provide good visual assistance to thepilot(s) to enable them to see the ground with sufficient visual acuitywithout making the aircraft excessively visible to outside observers. Itshould be further noted that, although embodiments are described forrotorcraft and VTOL aircraft that are operable to hover and performvertical takeoffs and landings, actual landings may not be vertical. Inmost examples of DVE landings, the approach is not vertical. However,the degradation of the visual environment is still a consequence of theuse of a rotating blade or wing surface to generate lift.

In some embodiments, additional landing lighting systems (referred toherein as a “dust-light” systems) can be installed singly or in pairs. Apair of lighting systems, one on each side of the cockpit can beparticularly useful for a typical cockpit crew comprising two pilots.Additional systems can also be provided for other crew members. Thesecan be either mounted to the airframe or handheld, for example, by acrew member located at an open doorway.

In some embodiments, the dust-lights are adapted for use in conjunctionwith NVGs. For example, the pilot can look through the NVGs out of thewindshield and cross-check beneath the goggles with the unaided eyethrough the chin bubble and/or out of the cockpit door or door windows;the pilot need not move their head, just their eyes. Minimal adaptationby the pilots is required; the new lighting system facilitates landingusing methods already familiar to trained pilots.

In exemplary embodiments, a system is provided for a Black Hawkhelicopter. The typical altitude below which dust envelopment occurs isabout 50 ft. Below this altitude, pilots may transition to lookingthrough the chin bubble during an approach. It will be apparent to oneof ordinary skill that the lower the altitude from which the lights areemployed, the less observable the aircraft will be from the surroundingenvironment. The lighting system need only provide enough light to aidthe pilot in seeing the landing zone visible through the chin bubble atdistances of 50 ft or less. In some embodiments, the lighting system isoptimized for use below 20 ft. Some observations by the inventor suggestthat operation at 20 ft and below provides sufficient light to provideadequate assistance to the pilot in heavy dust conditions.

In some embodiments visible light is used of sufficient intensity thatthe pilot can see the landing zone with the unaided eye below his NVGs.The light can be limited so as not to cause adverse reflections off dustduring the approach, particularly through the NVGs. In some embodimentslaser light can be used to supplement depth perception by providing anidentifiable point where the laser terminates on the surface. Thesurface immediately surrounding the laser termination point can beilluminated more generally, for example, using LED light sources orother low-level light sources as shown in FIG. 6. In some embodimentsthe laser light is omitted.

FIG. 6 shows the laser 602 as lines of visible light (e.g., green and/orred) with a termination point 604 shown as a sunburst symbol,exaggerated in size for the purpose of clarity. The LED beam width isrepresented by the dotted lines 606, and the illuminated surface isrepresented by the oval 608 surrounding the sunburst. Also shown is thenormal FOV 610 of the right-hand pilot through NVGs and the windshield.In some embodiments, the LEDs can be white and/or green for the unaidedeye, looking through the chin bubble below the NVGs. In someembodiments, the LEDs and/or laser can be infrared. When infrared lightis selected (e.g., for tactical reasons to reduce visibility), the pilotmakes all observations through the NVGs moving his head as necessary tolook through the NVGs and the chin bubble. In some embodiments, bothvisible and infrared light sources are provided, and the pilot canchoose light sources (and intensities) according to the needs of aparticular approach and/or landing. In some embodiments, a green and redlaser can be projected simultaneously to provide a beam that is visiblein and out of dust conditions. Table 1 provides a reference under whichconditions each illumination mode may be visible. The reference tovisibility for the LEDs pertains to the target surface, whereas thevisibility for the lasers pertains to the beam itself and/or the laserspot projected on the ground.

TABLE 1 Visibility Characteristics of Light Sources in DifferentConditions No Dust Dust Source Unaided Aided Unaided Aided LED White X XX X Green X X Infrared X X *Laser Red X X X Green X X X Infrared X X *Xindicates beam or spot is visible.

The specifications for the lasers and low level light sources can varyaccording to the needs of a particular aircraft and its landingcharacteristics. The output power can be made adjustable, either by thepilot directly, or with the assistance of some form of automatedintensity adjustment aided by a reflected light sensor. An automatedsystem can provide just enough light to provide a desired reflectedlight intensity returned to the aircraft with a maximum allowed levelbased on tactical considerations.

Any laser source providing a beam of appropriate wavelength and powercan be used. In the configuration of Example 1, three lasers, twovisible and one infrared are provided to accommodate different terrain,particulate, and tactical situations. The pilot can select whicheverprovides the best visibility subject to operating constraints.Typically, Class I lasers with an output power of 0.5-1 mW are suitable,although other powers can also be used. These lasers can besemiconductor lasers such as those used in laser-pointing andlaser-sighting applications. A typical green laser can have a wavelengthof about 532 nm; a typical red laser can have a wavelength of about 650nm; and a typical infrared laser can have a wavelength of about 830 nm,although these wavelengths can vary, and other wavelengths can be used.In some embodiments, the light from the laser is well-collimated suchthat the projected spot size on the ground is substantially constantduring approach. In some embodiments, the laser can be focused ordivergent with either fixed or adjustable focal length/divergence angle.A converging or diverging beam can be adjusted so that the projectedspot size on the ground changes with altitude and provides an additionalvisual cue to the pilot as to current altitude at short range whereconventional radar and barometric altimeters do not provide adequateprecision and accuracy. Astigmatism can also be deliberately used suchthat the projected spot has an aspect ratio that changes with altitude.For example, a cylindrical lens can provide a different effective focallength along one axis compared to a perpendicular axis. A round spotwill be projected at one altitude, and the spot will appear to beelongated along one axis of the other as the altitude deviates from thatgiving the round spot. In some embodiments, the focal lengths areadjusted to give a round spot at termination. In some embodiments twolasers can be aimed at different angles such that the separation oftheir projected spots varies with altitude. In some embodiments theprojected spots coincide when termination is reached.

The laser spots can provide useful visual cues as to the location of asurface that may be otherwise difficult to see through the particulatesand low-light conditions. A larger illuminated area can also be valuableto aid the pilot in landing at a particular target location, avoidingany local obstacles either on or above the ground. In some embodiments,the larger illuminated area is defined by a FOV around the laser spot,although it is also possible to point the laser spot and illuminatedarea independently toward different locations. Any low-levelillumination source can be used, including conventional landing lightsset at low power, although typical aircraft control systems are notconfigured to operate landing lights with the small FOV and lowintensities optimal for particulate environments and tactical orclandestine operations. The power levels for conventional landing lightsand dust-lights can be a more than an order of magnitude higher thanthose of an ideal dust-light system as can the desired FOV. Accordingly,a dedicated dust-light system can be a preferred implementation.

In some embodiments LEDs are used and can provide a suitable combinationof power level and controllability. In some embodiments an array of LEDsof each wavelength is provided. These can be arranged in any convenientgeometric configuration, the ring arrangement described in Example 1being only one of many possible configurations that would be apparent toone of skill in the art. The minimum total radiant power output shouldbe just sufficient to provide ground visibility to the pilot as hedescends below a particular altitude such as the 50 ft or 20 ftsuggested above for use with the Black Hawk helicopter. The maximumradiant power output should be limited so as not to provide more lightthan is necessary to assist the pilot, for example, no more than 2, 3,or 5 times the minimum radiant power output. For tactical use, theillumination level can be further controlled, either manually orautomatically so as to maintain only the minimum level needed at anygiven time and location. The illuminated area can be kept small to matchthe FOV of the pilot looking, for example, through the chin bubble. Thelight assembly can be further shrouded and aimed so that the light haslow visibility to any observer outside the FOV. In some embodiments thetotal power for each array of LEDs can be less than about 10 W or about20 W depending on the size and configuration of the particular aircraft.By comparison, the typical prior art landing lights and searchlightsoperate at or above about 600 W and 250 W respectively, and cannotreadily provide the low-level controlled intensities of the dust-lightsystem, even if it were possible to aim them in a useful direction forilluminating the surface seen through the chin bubble.

While light can penetrate particulates to some extent, a dust-lightsystem can only physically increase visibility to what may be observedduring equivalent dust conditions in daylight. Dust-light systems canaid in maintaining or establishing outside references in what wouldotherwise be total darkness, or particulate-entrained NVG-green-huedrotorwash.

In some embodiments, the aim of the dust-light system is fixed atinstallation based on an average pilot size and eye location. In someembodiments, fine tuning of the beam direction can be provided using asuitable gimble mount with two axes of adjustment. The adjustment canallow optimization of the aim of the dust-light system for a particularcombination of pilot, seat adjustments, and aircraft.

Example 1 A Multiwavelength Dust-Light Assembly with Both LEDs andLasers

FIGS. 7A-C show views of a design for an exemplary dust-light assembly.A 5-inch diameter mounting flange has a depth of about 2-3 in, and aweight of about 5 lbs. Three central recesses 702 are provided to mountred, green, and infrared semiconductor lasers. The surrounding recesses704-708 house the LEDs by type in concentric rings. From inner to outerrings the order is infrared 704, green 706, and white 708. The chamferedouter ring is designed to collimate the LED beam so that it onlyilluminates a target area consistent with the FOV available through asingle chin bubble at 50 ft and below. In addition, the outer ringserves to shroud the light source from outside observation in a tacticalenvironment. The inner face of the dust-light is further protected by ascratch-proof, non-reflective, tinted glass which also aids in reducingoutside observation.

The outer flange of the unit mounts flush to the aircraft exterior to afixed flange using four bolts with thread locking compound, and furthersealed around the perimeter of the unit using standard nonpermanentcompound (e.g., PRO-SEAL® made by Proseal, Adlington, Cheshire UK). Themounting configuration would be similar to that of the Electro-OpticMissile Sensors (EOMS) of the Common Missile Warning System (CMWS),shown in FIG. 8.

Power can be provided to the back of the unit via a plug connection 802,supplying 28 VDC from the number 2 DC primary bus.

Example 2 Aircraft Integration

In order to adapt a dust-light system to a specific airframe, theergonomics of the cockpit layout and nose section of the aircraft mustbe taken into account. One of the primary considerations is the FOV fromthe pilots' perspective through the chin bubble. In this example,integration is described for a Black Hawk helicopter. The chin bubble ina Black Hawk is reasonably sized, but does not provide much forwardlooking capability. Rather it provides more lateral and downwardvisibility as shown in FIG. 9.

A superimposed ring 902 shows the approximate center of the FOV throughthe chin bubble for the right-hand pilot. When the aircraft is on theground, the fixed dust-lights can be aimed at their respective chinbubble FOV center. The seat position from which the image in FIG. 9 wascaptured is full aft, and mid-position height, in accordance with thedesign eye point for a 70-inch tall pilot. The design eye point for theUH-60 in accordance with the Army field manual [Army 2007A], is to havethe ground in view beginning at 12 ft from the nose (with the aircrafton the ground). Measuring from the approximate pilot's eye position, theviewing angle ranges are found to be approximately 42-49° down andapproximately 18-22° outward. The seats are capable of adjusting a totalof 5 inches fore/aft and up/down. The anti-torque pedals 904 are shownfull forward, but can be adjusted fore/aft a total of 6.5 inches. TheFOV from the left pilot seat is approximately the same. Thesemeasurements cannot account for every possible combination of height,seat position, pedal adjustment, and anthropometric variability, butthey are valid for the majority of military aviators given theconstraints of the HH-60L cockpit.

Visible surface area outside the chin bubble from the pilot'sperspective is approximately 6.5 ft² with the aircraft on the ground.This was measured by placing rigid plastic sheets marked with squarefoot gridlines beneath the chin bubble and estimating the visible areaas shown in FIG. 10A. Following the inverse square law in regard toarea, as shown in FIG. 10B, it is evident that visible area increases bymultiplying by the square of the radius (distance) r, where r is thedistance from the pilot's eyes to the ground along the central FOV axis,i.e., 9 ft.

At 50 ft altitude, measured from the Earth's surface to the radaraltimeter antenna, the pilot's eyes are 54.5 ft above the surface; 54.5ft divided by 9 ft equals 6.05 r, squaring 6.05 equals 36.67, andmultiplying this result by 6.5 ft² yields 238.35 ft². Therefore, thearea visible through the chin bubble at 50 ft altitude is approximately238.35 ft². This is equivalent to a square surface area of 15.44×15.44ft, or a circular surface area 17.42 ft in diameter. Note these areestimates based on specific eye point with the aircraft at a levelattitude. Shifting of the pilots head and changing the aircraft pitchattitude can alter this figure drastically. But these estimates can beused to establish a baseline figure for a preliminary design.

The dust-light can be located in an area which can readily provide theproper positioning for the system to work effectively, while minimizingthe required modification to existing airframe structure. Two primarylocations 1102 and 1104 are identified in FIG. 11. Location 1102requires minimal modification due to an existing maintenance accesspanel. Only the panel itself would require modification; the dust-lightwould be directly mounted to the panel. Alternatively, the dust-lightcan be provided as a direct bolt-on replacement for the access panel.The addition of the dust-light would not interfere with normalmaintenance actions associated with this location. FIG. 12 shows thearea located directly behind the panel at location 1102. Flight controllinkages 1202 are visible behind the panel, but are several inches fromthe panel itself leaving sufficient room for the dust-light systemhardware. Internal space is, however, more limited than at location1104. The more significant issue that complicates using location 1102 isthat the dust-light must project at a significant angle relative to thecenterline axis of the mounting hole in order for it to be effective.This would require either angling the face of the dust light, or addingan angled-mirror modification to project the beam properly.

Location 1104 offers placement of the dust-light that is more alignedwith the required aiming direction for the dust-light. The drawback isthat location 1104 would require more airframe modifications thanlocation 1102. The modifications required would consist of cutting sheetmetal and drilling using a circular template, which may or may not beconsidered a unit-level maintenance action, depending on theorganizational resources. FIG. 13 shows the space behind location 1104from a vantage point within the cockpit and behind the anti-torquepedals, with an interior panel removed. The intended location of thesheet metal modification and dust-light installation is indicated by thecircle 1302. The control rod running through the circle is offset aboveand to the side of the mounting site, and will not cause interference.

If the AN/AVR-2B Laser Warning System is installed on the aircraft, theassociated mounting structure can also serve as a potential location1402 for the dust-lights as shown in FIG. 14. Location 1402 iscompatible with achieving the proper beam angle and is practical in thatit only requires modification to an existing modular component,requiring no direct airframe modification.

The dust-lights can be controlled via a rocker switch mounted on thecollective. The collective is shown in FIG. 15, depicting where severalexisting light functions can be controlled. One control option is toreplace the existing searchlight rocker switch 1502 with a split rockerswitch, one side for the dust-light modes, and the other remaining sidefor searchlight functions. This placement would be a seamlessintegration, requiring no change to the pilots' natural hand placement.The only change would be in tactile identification of the appropriateswitch. Given the vast diversity of switches available, and the amplesize of the Black Hawk collective control head, it would be a suitableoption.

Example 3 An Approach Sequence

FIGS. 16 and 17 show a photo sequence taken during a single approachinto a heavy dust landing zone. Looking closely, one can observe thedust formation and visibility in relation to altitude. FIG. 16A showsthe aircraft at 19 ft above ground level; the dust cloud is beginning tocome into view from the cockpit.

The dust obscures the chin bubble first as the rotorwash begins to shearmaterial from the surface. As the helicopter closes on a landingsurface, a surrounding wall of dust forms, and the FOV through the chinbubble begins to clear, as shown in FIG. 16B, where the aircraft is at 9ft above ground level. This is consistent with the dust simulationmodels shown in [Phillips 2009] that are reproduced in FIGS. 3-5. As thehelicopter continues to descend, the surrounding dust-entrained air massbegins to circulate through the rotor system, further degradingvisibility as shown in FIG. 17A, where the aircraft is at 3 ft aboveground level.

However, visibility continues to improve through the chin bubble all theway to termination on the ground illustrated in FIG. 17B. Theillustrated approach was made to a specific point with low touchdownspeed resulting in about a one foot roll-out. The wind was fromapproximately 270° at approximately 20 knots and the approach directionwas 310°. Conducting this approach at night using NVGs would have beenfar more challenging, and could have easily resulted in a go-around. Thearea beneath the chin bubble would have been completely dark and wouldhave offered no outside reference to the unaided eye; the pilot wouldhave had to repeatedly move his head downward to look through the chinbubble using his NVGs to gain reference during this critical stage.Making such head movements is not ideal and potentially unsafe.

It will be understood that the descriptions of one or more embodimentsof the present invention do not limit the various alternative, modifiedand equivalent embodiments which may be included within the spirit andscope of the present invention as defined by the appended claims.Furthermore, in the detailed description above, numerous specificdetails are set forth to provide an understanding of various embodimentsof the present invention. However, one or more embodiments of thepresent invention may be practiced without these specific details. Inother instances, well known methods, procedures, and components have notbeen described in detail so as not to unnecessarily obscure aspects ofthe present embodiments.

REFERENCES

-   [Army 2007A] Field Manual 3-04.203 Fundamentals of Flight, Dept. of    the Army, Washington, D.C.-   [Army 2007B] Training Circular 1-237Aircrew Training Manual, Utility    Helicopter, H-60 Series, Dept. of the Army, Washington, D.C.-   [Army 2008] Army Regulation 95-1 Aviation Flight Regulations, Dept.    of the Army, Washington, D.C.-   [Army 2010] Technical Manual 1-1520-253-10 Operator's Manual for    HH-60L Helicopter, Dept. of the Army, Washington, D.C.-   [Army 2013] Training Circular 3-04.33 Aircrew Training Manual,    Utility Helicopter, H-60 Series, Dept. of the Army, Washington, D.C.-   [Harrington 2010] Harrington, W. et al., “3D-LZ Brownout Landing    Solution,” American Helicopter Society 66^(th) Annual Forum,    Phoenix, Ariz., Ann. Forum Proc.—AHS, 66 (2010), 983-1001.-   [Phillips 2009] Phillips, C. & Brown, R. E., (2009): “Eulerian    Simulation of the Fluid Dynamics of Helicopter Brownout,” Journal of    Aircraft, 46 (2009), 1416-29, doi: 10.2514/1.41999.

What is claimed is:
 1. An aircraft lighting system comprising one ormore lighting units mounted to an aircraft, the aircraft operable tohover near a landing zone, wherein each lighting unit is operable toprovide adjustable illumination to an illuminated area in the landingzone, and wherein each lighting unit has a radiant power output betweena minimum and a maximum, wherein the minimum radiant power output isjust sufficient to allow a pilot to distinguish features in the landingzone when below a first altitude wherein the downwash from the aircraftrotors, propellers, or engines begins to raise particulates from thelanding zone and continues to be just sufficient to allow the pilot todistinguish features as the pilot descends from the first altitude totermination at the landing zone, and wherein the maximum radiant poweroutput is less than about five times the minimum radiant power output.2. The aircraft lighting system of claim 1, wherein each lighting unitcomprises a plurality of lighting elements emitting light at a pluralityof wavelengths.
 3. The aircraft lighting system of claim 2, wherein theplurality of wavelengths can produce white, green, and infraredillumination.
 4. The aircraft lighting system of claim 2, wherein theplurality of lighting elements comprise light emitting diodes (LEDs). 5.The aircraft lighting system of claim 1, wherein the illuminated area ofthe landing zone is approximately equal in size and shape to the fieldof view of a pilot or crew member looking through a chin bubble or aside door or window of the aircraft.
 6. The aircraft lighting system ofclaim 5, further comprising a shroud, wherein the shroud is operable tolimit the emission of light to a direction toward the illuminated areaof the landing zone.
 7. The aircraft lighting system of claim 1, whereineach lighting unit further comprises one or more lasers aimed generallyin the direction of the illuminated area of the landing zone.
 8. Theaircraft lighting system of claim 7, wherein each lighting unitcomprises a plurality of lasers emitting light at a plurality ofwavelengths.
 9. The aircraft lighting system of claim 8, wherein theplurality of wavelengths comprise red, green, and infrared.
 10. Theaircraft lighting system of claim 8, wherein the plurality of laserscomprise semiconductor lasers each having a power output of between 0.1mW and 1.0 mW.
 11. The aircraft lighting system of claim 7, wherein theone or more lasers create a pattern onto the landing zone which changesas a function of altitude.
 12. The aircraft lighting system of claim 11,wherein a spot projected from the one or more lasers on the landing zonechanges size as a function of altitude.
 13. The aircraft lighting systemof claim 11, wherein a spot projected from the one or more lasers ontothe landing zone changes shape as a function of altitude.
 14. Theaircraft lighting system of claim 11, wherein two spots projected fromthe one or more lasers onto the landing zone have a separation whichchanges as a function of altitude.
 15. The aircraft lighting system ofclaim 1, wherein the particulates comprise dust or sand.
 16. Theaircraft lighting system of claim 1, wherein the particulates comprisewater, snow, or ice.
 17. A method for illuminating a landing zonecomprising providing one or more lighting units to an aircraft, theaircraft operable to hover near the landing zone, wherein each lightingunit is operable to provide adjustable illumination to the landing zone,and wherein each lighting unit has a power between a minimum and amaximum, wherein the minimum power is just sufficient to allow a pilotto distinguish features in the landing zone from below a first altitudewherein the downwash from the aircraft rotors, propellers, or enginesbegins to raise particulates from the landing zone and continues to bejust sufficient to allow the pilot to distinguish features as the pilotdescends from the first altitude to termination at the landing zone, andwherein the maximum power is less than about five times the minimumpower; during an approach or landing of the aircraft, leaving thelighting units off until the aircraft descends below the first altitude,and turning the lighting units on below a second altitude wherein thesecond altitude is below the first altitude.
 18. The method of claim 17,wherein the second altitude is about 20 ft.